Spurious Resonances
All quartz crystals have spurious resonances (unwanted resonance) besides the main resonance frequency. They are represented in the equivalent circuit diagram(Fig.5) by additional resonant circuits in parallel with R1, L1, C1. The ratio of spurious resonance resistance R NW to resonance resistance Rr of the main wave is generally specified in the attenuation constant dB and designated spurious attenuation a NW : a NW =-20 · lg
For oscillator crystals 3 to 6 dB are normally sufficient. For filter crystals attenuations greater than 40 dB are often required. This can only be achieved by special design techniques and involves the use of very small values of the dynamic capacity C1.
The achievable attenuation decreases with higher frequency and with higher orders of overtone. It is found generally that plano-parallel quartz resonators have better spurious attenuation than plano-convex resonators. In specifying spurious resonance parameters it is necessary to give an indication of both the acceptable attenuation level desired and their frequencies relative to main resonance frequency.
Resonance in the region of +40 to +150 KHZ for plano parallel resonators biconvex or plano-convex resonators.
In the passive measurement method indicated above, spurious resonance attenuations up to 20 to 30 dB can be measured. With higher attenuations, Co compensation is necessary.